Separation of unsaturated hydrocarbons by extractive distillation

ABSTRACT

The separation of alkadienes from close-boiling alkenes by extractive distillation employs as solvent either N-(β-mercaptoethyl)-2-pyrrolidone alone, or a mixture of N-(β-mercaptoethyl)-2-pyrrolidone and either N-methyl-2-pyrrolidone or cyclohexanol, or a mixture of cyclohexanol and tetraethylene glycol. The separation of cycloalkadines from close-boiling alkadienes by extractive distillation employs N-(β-mercaptoethyl)-2-pyrrolidone as solvent.

BACKGROUND OF THE INVENTION

This invention relates to the separation of alkadienes (aliphaticdiolefins) from close-boiling alkenes (aliphatic monoolefins) byextractive distillation. In another aspect, this invention relates tothe separation of cycloalkadienes (cyclodiolefins) from close-boilingalkadienes (aliphatic diolefins) by extractive distillation.

Extractive distillation is a well known technique for separatingmixtures of components having a relative volatility close to unity(i.e., having nearly equal volatility and having nearly the same boilingpoint). It is difficult to separate the components of such mixtures byconventional fractional distillation. In extractive distillation, asolvent is introduced into a distillation column above the entry pointof the feed mixture which is to be separated. The solvent affects thevolatility of the higher boiling feed component(s) sufficiently tofacilitate the separation of the various feed components by distillationand exits with the bottoms fraction, as has been described in thearticle entitled "Extractive Distillation Saves Energy" by IanSucksmith, Chemical Engineering, Jun. 28, 1982, pages 91-95. Otherliterature sources on extractive distillation techniques include the"Handbook of Separation Techniques for Chemical Engineers" by Philip A.Schweitzer, McGraw-Hill Book Company, 1979, pages 1-135 to 1-143; andPerry's Chemical Engineers Handbook, 6th Edition, McGraw-Hill BookCompany 1984, pages 13-53 to 13-57.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a process for separatingalkadienes from close-boiling alkenes by extractive distillationemploying a selective solvent (also referred to as extractant orentrainer). It is another object of this invention to provide a processfor separating cycloalkadienes from alkadienes by extractivedistillation employing a selective solvent. Other objects and advantageswill be apparent from the detailed description of the invention and theappended claims.

In accordance with this invention, a process for separating at least onealkadiene containing 4-9 carbon atoms per molecule from at least oneclose-boiling alkene by extractive distillation of a feed comprising(preferably consisting essentially of) said at least one alkadiene andsaid at least one alkene employs a solvent consisting essentially of atleast one liquid selected from the group consisting ofN-(β-mercaptoethyl)-2-pyrrolidone, mixtures of N-methyl-2-pyrrolidoneand N-(β-mercaptoethyl)-2-pyrrolidone, mixtures of cyclohexanol andN-(β-mercaptoethyl)-2-pyrrolidone, and mixtures of cyclohexanol andtetraethylene glycol.

Also, in accordance with this invention, a process for separating atleast one cycloalkadiene containing 5-9 carbon atoms per molecule fromat least one close-boiling alkadiene by extractive distillation of afeed comprising (preferably consisting essentially of) said at least onealkadiene and said at least one alkene employs a solvent consistingessentially of N-(β-mercaptoethyl)-2-pyrrolidone.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 illustrates the extractive distillation process of thisinvention.

DETAILED DESCRIPTION OF THE INVENTION

In an extractive distillation process, an agent (called "solvent" or"extractant" or "entrainer") is added to a feed mixture of components tobe separated so that the relative volatilities of the components of themixture are changed such that a sufficient difference in volatility ofthe components results and effective separation by distillation becomespossible. The added solvent is usually chosen so as to exhibit high"selectivity" regarding the components to be separated. Selectivity is aterm related to the change in volatilities of components in the mixturecaused by the presence of the solvent. The larger the difference inrelative volatility of the components in the mixture, the easier theseparation of the components by fractional distillation becomes.Therefore, a solvent of high selectivity is a solvent which causes greatdifferences between the relative volatilities of the components in amixture, and will allow for the separation of components in a mixturewith fewer distillation stages, lower amount of reflux and higherproduct purity. The term "close-boiling" as used herein, means that thefeed components have nearly the same boiling point at atmosphericpressure.

In the first embodiment of this invention, any hydrocarbon feed whichcomprises at least one alkadiene containing 4-9 carbon atoms permolecule and at least one close-boiling alkene (preferably containing4-10 carbon atoms per molcule) can be used. Preferably, the boilingpoints (at atmospheric pressure conditions, i.e., at about 1 atm.) ofthe alkadiene(s) and of the alkene(s) to be separated by extractivedistillation process of this invention, are in the range of from about15° F. to about 400° F., more preferably about 100°-350° F. Generally,the boiling points of the alkadiene(s) and of the alkene(s) differ byabout 0.2°-10° F. (preferably about 0.5°-5° F.), at about 1 atm.Preferably, the alkadiene content in the feed is about 5-95 weight-%(more preferably about 20-80 weight-%), and the alkene content is about5-95 weight-% (more preferably about 20-80 weight-%).

Non-limiting examples of suitable feed alkadienes are 1,2-butadiene,1,3-butadiene, isoprene, 1,2-pentadiene, 1,3-pentadiene, 2,4-pentadiene,1,2-hexadiene, 1,3-hexadiene, 1,4-hexadiene, 1,5 hexadiene,2,4-hexadiene, 2-methyl-1,2-pentadiene, 2-methyl-1,3-pentadiene,1,2-heptadiene, 1,3-heptadiene, 1,4-heptadiene, 2-methyl-1,2-hexadiene,3-methyl-1,2-hexadiene, 2-methyl-1,3-hexadiene, 3-methyl-1,3-hexadiene,1,2-octadiene, 1,3-octadiene, 1,4-octadiene, 1,5-octadiene,2-methyl-1,2-heptadiene, 3-methyl-1,2-heptadiene,2-methyl-1,3-heptadiene, 3-methyl-1,3-heptadiene, 3-ethyl-1,2-hexadiene,2-methyl-3-ethyl-1,3-pentadiene, 1,2-nonadiene, 1,3-nonadiene,2,4-nonadiene, 2-methyl-1,2-octadiene, 3-methyl-1,2-octadiene,3-methyl-1,3-octadiene, 3-ethyl-1,3-heptadiene, 3-ethyl-1,4-heptadiene,and mixtures thereof; in particular 1,5-hexadiene or2-methyl-1,3-pentadiene.

Non-limiting examples of suitable alkenes are 1-butene, 2-butene,2-methylpropene (isobutene), 1-pentene, 2-pentene, 2-methyl-1-butene,1-hexene, 2-hexene, 3-hexene, 2-methyl-1-pentene, 2-methyl-2-pentene,3-methyl-1-pentene, 2,3-dimethyl-1-butene, 1-heptene, 2-heptene,3-heptene, 2-methyl-1-hexene, 2-methyl-2-hexene, 3-methyl-2-hexene,3-methyl-3-hexene, 3,3-dimethyl-1-pentene, 1-octene, 2-octene, 3-octene,2-methyl-1-heptene, 1-nonene, 2-nonene, 3-nonene, 1-decene, 2-decene,and the like, and mixtures thereof; preferably 2-heptene or 1-hexene.

In the second embodiment of this invention, any hydrocarbon feed whichcontains at least one cycloalkadiene containing 5-9 carbon atoms permolecule and at least one close-boiling alkadiene (preferably containing4-9 carbon atoms per molecule) can be used. Preferably, the boilingpoints (at atmospheric pressure conditions) of the cycloalkadiene(s) andof the alkadiene(s) to be separated by the extractive distillationprocess of this invention, are in the range of about 80° F. to about400° F., more preferably 100°-350° F. Generally, the boiling points ofthe cycloalkadiene(s) and of the alkadiene(s) differ by about 0.2°-10°F. (preferably about 0.5°-5° F.), at about 1 atm. Preferably, thecycloalkadiene content in the feed is about 5-95 weight-% (morepreferably about 20-80 weight-%), and the alkadiene content in the feedis about 5-95 weight-% (preferably about 20-80 weight-%).

Non-limiting examples of suitable alkadienes are listed above.Non-limiting examples of suitable cycloalkadienes are1,2-cyclopentadiene, 1,3-cyclopentadiene, 1,2-cyclohexadiene,1,3-cyclohexadiene, 1-methyl-1,2-cyclopentadiene,2-methyl-1,2-cyclopentadiene, 3-methyl-1,2-cyclopentadiene,1-methyl-1,3-cyclopentadiene, 2-methyl-1,3-cyclopentadiene,3-methyl-1,3-cyclopentadiene, 1,2-cycloheptadiene, 1,3-cycloheptadiene,1,4-cycloheptadiene, 1-methyl-1,2-cyclohexadiene,1-methyl-1,3-cyclohexadiene, 2-methyl-1,3-cyclohexadiene,2-methyl-1,4-cyclohexadiene, 1-ethyl-1,2-cyclopentadiene,1-ethyl-1,3-cyclopentadiene, 2-ethyl-1,3-cyclopentadiene,1,2-dimethyl-1,2-cyclopentadiene, 1,2-dimethyl-1,3-cyclopentadiene,1,3-dimethyl-1,3-cyclopentadiene, 1-methyl-1,3-cyclohexadiene,1,2-dimethyl-1,3-cyclohexadiene, 1,3-dimethyl-1,3-cyclohexadiene,1,2,3-trimethyl-1,3-cyclohexadiene, and mixtures thereof; in particular1,3-cyclohexadiene.

Any suitable weight ratio of the solvent to the hydrocarbon containingfeed mixture can be employed. Generally, the solvent to feed weightratio is in the range of from about 1:1 to about 40:1, preferably in therange of from about 5:1 to about 20:1.

The solvent employed in the first process of this invention can beN-(β-mercaptoethyl)-2-pyrrolidone alone, or a mixture ofN-(β-mercaptoethyl)-2-pyrrolidone and either N-methyl-2-pyrrolidone orcyclohexanol, or a mixture of cyclohexanol and tetraethylene glycol. Inthe second process of this invention, the solvent consists essentiallyof N-(α-mercaptoethyl)-2-pyrrolidone. When one of the above mixtures isused as a solvent, the weight ratio of one solvent component to theother solvent component generally is in the range of about 1:20 to about20:1, preferably about 1:5 to about 5:1. The solvent components areeither commercially available or can be prepared by known methods. Thepreparation of N-(β-mercaptoethyl)-2-pyrrolidone is described in U.S.Pat. Nos. 4,954,224 and 4,955,468.

Any suitable reflux ratio (i.e., the weight ratio of the portion ofcondensed vapor which is returned to the distillation column to theportion of condensed vapor which is withdrawn as distillate product) canbe employed in the extractive distillation process of this invention.Generally the reflux ratio is in the range of from about 0.1:1 to about100:1, preferably in the range of from about 0.5:1 to about 50:1, morepreferably in the range of from about 1:1 to about 20:1.

Any suitable feed entry location can be selected. Generally the feedentry location is in the range of from about 2 to about 70 percent ofthe total height of the packed or trayed column, measured upward fromthe bottom of the column, preferably in the range of from about 5 toabout 60 percent, more preferably in the range of from about 7 to about70 percent.

Any suitable solvent entry location can be selected. Generally thesolvent entry location is in the range of from about 50 to about 99percent of the total height of the packed or trayed column (i.e., withinthe upper half of the column), preferably in the range of from about 70to about 99 percent, more preferably in the range of from about 80 toabout 99 percent.

Any suitable temperature in the reboiler vessel (containing primarilythe higher boiling feed components and the solvent) can be employed. Thetemperature is generally in the range of from about 100° to about 400°F., preferably in the range of from about 150° to about 320° F. Theextractive distillation column is generally heated (more near thebottom, and less near the top). Generally, the temperature at the top ofthe column where the vapor exits into the condenser is in the range offrom about 100° to about 300° F., preferably in the range of from about150° to about 250° F. Solvent and feed are generally preheated(generally to a temperature close to the column temperature of thecorresponding entry point) before they are introduced into the column.Any suitable pressure can be employed during the extractivedistillation. Generally the pressure is about 5 to about 100 psig,preferably about 8 to about 20 psig.

In the first embodiment of this invention, the overhead distillateproduct (withdrawn from the top of the column) contains a smaller volumepercentage of the alkadiene(s) than the feed and a larger volumepercentage of alkene(s) than the feed, and the bottoms product (aportion of which can be reheated and recycled to the lower portion ofthe column) contains a larger volume percentage of the alkadiene(s) thanthe feed and a smaller volume percentage of the alkene(s) than the feed.In the second embodiment of this invention, the overhead distillateproduct contains a smaller percentage of the cycloalkadiene(s) than thefeed and a larger percentage of the alkadiene(s) than the feed, and thebottoms product contains a larger volume percentage of thecycloalkadiene(s) than the feed and a smaller volume percentage of thealkadiene(s) than the feed. The bottoms product contains essentially allof the added solvent, which can be separated from the other bottomsproduct components by distillation or other suitable separating meansand then be recycled to the extractive distillation column.

Any suitable total column height, packed column height, column diameterand number of trays in the extraction distillation column can beemployed. The exact dimensions and column designs depend on the scale ofthe operation, the exact feed composition, the exact solventcomposition, the desired recovery and degree of purity of the variousproduct, and the like, and can be determined by those having ordinaryskills in the art.

The invention can be better understood by reference to FIG. 1 and thefollowing description of a preferred embodiment of the invention. In thefirst embodiment of this invention, the feed mixture comprisingalkadiene(s) and close-boiling alkene(s) is introduced through conduit10 to a fractionation zone such as multi-stage distillation column 12.The temperature of the feed mixture flowing through conduit 10 can beadjusted as needed by controlling heat exchanger 14 so as to add heat toor remove heat from the feed mixture. Solvent from solvent storage 6 isintroduced to distillation column 12 through conduit 8, and an overheadstream enriched in alkene(s) is withdrawn from an upper portion ofdistillation column 12 through conduit 16. This overhead stream can becompletely passed to storage or to other processing units or, as isoften the case, the overhead stream can be partially or totallycondensed, with a portion thereof being returned to the fractionationzone as reflux. The overhead stream passing through conduit 16 iscondensed in condenser 22 to yield a condensed overhead stream. Aportion of the condensed overhead stream can be returned to distillationcolumn 12 as reflux through conduit 18, while the remainder of thecondensed overhead stream is yielded as product or passed to otherprocessing units through conduit 20.

A bottoms stream is withdrawn from a lower portion of the fractionationzone represented by distillation column 12 through conduit 24. A portionof the fluids withdrawn from the bottom of distillation column 12 may beheated and returned to distillation column 12. For example, a portion ofthe bottoms product stream can be withdrawn through conduit 25, heatedin reboiler 26 and then passed back to a lower portion of distillationcolumn 12 through conduit 27.

Operating conditions in heat exchanger 14, condenser 22 and reboiler 26can be controlled and interfaced with solvent flow through conduit 8,feed mixture flow through conduit 10, reflux flow through conduit 18 andbottoms stream flow through conduit 24 such that the feed mixtureintroduced into distillation column 12 will be fractionated to yield anoverhead stream which is enriched in alkene(s) and a bottoms streampredominantly comprising the alkadiene(s) and the solvent.

The bottoms stream passing through conduit 24 can be passed to storage,used in other processes or, preferably, passed to another fractionationzone, such as distillation column 29. Any adjustments to the temperatureof the bottoms stream passing through conduit 24 necessary for efficientfractionation in distillation column 29 can be made by appropriatelyadjusting heat exchanger 28. An overhead stream predominantly comprisingalkadiene(s) is withdrawn from an upper portion of distillation column29 through conduit 30. This overhead stream can be at least partiallycondensed in condenser 32. A portion of the overhead stream withdrawnfrom condenser 32 can be returned through conduit 34 as reflux fordistillation column 29, with the remainder of the overhead stream beingwithdrawn as product, i.e., alkadiene(s) of high purity (preferablyhigher than 95%), through conduit 36.

A bottoms stream predominantly comprising the solvent is withdrawn froma lower portion of distillation column 29 through conduit 38. A portionof this bottoms stream is preferably routed back to solvent storage 6and then recycled to distillation column 12, while another portion ofthe bottoms stream is heated in a reboiler (not shown) and returned tothe lower portion of column 29. From time to time, impurities which maybuild up in the solvent can be removed from the system by removing asmall purge stream through conduit 40. Solvent lost through the purgestream or through other processing losses may be made up by a makeupstream passing through conduit 42 and into solvent storage 6.

The above description of FIG. 1 and the various process steps can alsobe applied to the second embodiment of this invention which employs afeed mixture of cycloalkadiene(s) and alkadiene(s) by replacing"alkadiene(s)", wherever it occurs in the description, with"cycloalkadiene(s)", and by replacing "alkene(s)", wherever it occurs inthe description, with "alkadiene(s)". Thus, in the second embodiment ofthe process of this invention, alkadiene(s) accumulate in the overheadstream and "cycloalkadienes" accumulate in the bottoms product.

The following examples are presented to further illustrate the inventionand are not to be considered unduly limiting the scope of thisinvention.

EXAMPLE I

This example demonstrates the use of various solvents in the extractivedistillation of an alkadiene/alkene feed.

To a hydrocarbon mixture of 50 weight-% 1,5-hexadiene and 50 weight-%1-hexene was added an extractive solvent at various solvent: feed weightratios. The total mixture (including the extractive solvent) was heatedunder reflux conditions for about 20-30 minutes in a distillation flaskequipped with a reflux condenser. Then a small sample was withdrawn bymeans of a septum from the flask containing the liquid phase of theequilibrium system, and a sample of the condensed vapor was withdrawn bymeans of a septum located just below the reflux condenser. Both sampleswere analyzed, and the mole fractions of 1,5-hexadiene and 1-hexene inthe liquid phase and in the vapor phase were determined by means of agas chromatograph. The relative volatility R¹ was calculated as follows:##EQU1## wherein Y1 and Y2 are the mole fractions of 1-hexene and1,5-hexadiene, respectively, in the vapor phase; and X1 and X2 are themole fractions of 1-hexene and 1,5-hexadiene, respectively, in theliquid phase.

The following solvents were tested: N-(β-mercaptoethyl)-2-pyrrolidone(MEP), N-methyl-2-pyrrolidone (NMP), a mixture of 90 weight-% MEP and 10weight-% NMP; cyclohexanol (CHOL), a mixture of 90 weight-% MEP and 10weight-% CHOL, tetraethylene glycol (TEG), and a mixture of 75 weight-%TEG and 25 weight-% CHOL. Test results are summarized in Table I.

                  TABLE I                                                         ______________________________________                                                                  Relative                                            Solvent:Feed  Added       Volatility                                          Weight Ratio  Solvent     R.sup.1                                             ______________________________________                                        1:1           MEP         1.12                                                1:1           NMP         1.07                                                1:1           MEP + NMP   1.48                                                1:1           CHOL        0.95                                                1:1           MEP + CHOL  1.4                                                 1:1           TEG         0.94                                                1:1           TEG + CHOL  0.97                                                3:1           MEP         1.43                                                3:1           NMP         1.15                                                3:1           MEP + NMP   1.85                                                3:1           CHOL        0.97                                                3:1           MEP + CHOL  1.8                                                 3:1           TEG         1.0                                                 3:1           TEG + CHOL  1.07                                                5:1           MEP         1.67                                                5:1           NMP         1.15                                                5:1           MEP + NMP   2.08                                                5:1           CHOL        0.98                                                5:1           MEP + CHOL  --                                                  5:1           TEG         1.05                                                5:1           TEG + CHOL  1.09                                                7:1           MEP         2.37                                                7:1           NMP         1.17                                                7:1           MEP + NMP   2.02                                                7:1           CHOL        1.0                                                 7:1           MEP + CHOL  --                                                  7:1           TEG         1.09                                                7:1           TEG + CHOL  1.12                                                ______________________________________                                    

Based on the test results in Table I, it is concluded thatN-(β-mercaptoethyl)-2-pyrrolidone, alone or in admixture withN-methyl-2-pyrrolidone (NMP) or cyclohexanol, will be more effectivethan NMP alone or cyclohexanol alone as solvent in the separation of C₄-C₉ alkadienes from close-boiling alkenes by extractive distillation. Itis also concluded that mixtures of tetraethylene glycol and cyclohexanolwill be more effective than tetraethylene glycol alone or cyclohexanolalone as solvent in the separation of C₄ -C₉ alkadienes fromclose-boiling alkenes by extractive distillation.

EXAMPLE II

This example demonstrates the use of N-(β-mercaptoethyl)-2-pyrrolidone(MEP) as solvent in the extractive distillation of acycloalkadiene/alkadiene feed.

Tests were carried out substantially in accordance with the proceduredescribed in Example I, except that a mixture of 50 weight-%1,3-cyclohexadiene and 50 weight-% 2-methyl-1,3-pentadiene was used ashydrocarbon feed. Two solvents were employed: MEP and NMP(N-methyl-2-pyrrolidone). The relative volatility R² was calculated asfollows: ##EQU2## wherein Y3 and Y4 are the mole fractions of2-methyl-1,3-pentadiene and 1,3-cyclohexadiene, respectively, in thevapor phase; and X3 and X4 are the mole fractions of2-methyl-1,3-pentadiene and 1,3-cyclohexadiene, respectively, in theliquid phase. Test results are summarized in Table II.

                  TABLE II                                                        ______________________________________                                                                 Relative                                             Solvent:Feed    Added    Volatility                                           Weight Ratio    Solvent  R.sup.2                                              ______________________________________                                        1:1             MEP      1.32                                                 1:1             NMP      1.23                                                 3:1             MEP      1.47                                                 3:1             NMP      1.30                                                 5:1             MEP      1.52                                                 5:1             NMP      1.32                                                 7:1             MEP      1.55                                                 7:1             NMP      1.38                                                 ______________________________________                                    

Based on the test results in Table II, it is concluded thatN-(β-mercaptoethyl)-2-pyrrolidone (MEP) will be more effective thanN-methyl-2-pyrrolidone (NMP) as solvent in the separation of C₅ -C₉cycloalkadienes from close-boiling alkadienes by extractivedistillation.

Reasonable variations, modifications and adaptations for various usagesand conditions can be made within the scope of the disclosure and theappended claims, without departing from the scope of this invention.

That which is claimed is:
 1. A process for separating at least onealkadiene containing 4-9 carbon atoms per molecule from at least onealkene containing 4-10 carbon atoms per molecule comprising extractivedistillation of a feed which consists essentially of said at least onealkadiene and said at least one alkene employing a solvent consistingessentially of N-(β-mercaptoethyl)-2-pyrrolidone;wherein said processproduces (i) an overhead product which contains a smaller volumepercentage of said at least one alkadiene and a larger volume percentageof said at least one alkene than said feed, and (ii) a bottoms productwhich contains said solvent and a larger volume percentage of said atleast one alkadiene and a smaller volume percentage of said at least onealkene than said feed; and wherein said at least one alkadiene isseparated from said solvent contained in said bottoms product.
 2. Aprocess in accordance with claim 1, wherein said at least one alkadieneis 1,5-hexadiene and said at least one alkene is 1-hexene.
 3. A processin accordance with claim 1, wherein said feed boils at a temperature inthe range of about 15° F. to about 400° F., at atmospheric pressureconditions.
 4. A process in accordance with claim 1, wherein the weightratio of said solvent to said feed is in the range of about 1:1 to about40:1.
 5. A process for separating at least one cycloalkadiene containing5-9 carbon atoms per molecule from at least one alkadiene containing 4-9carbon atoms per molecule comprising extractive distillation of a feedwhich consists essentially of said at least one cycloalkadiene and saidat least one alkadiene employing a solvent consisting essentially ofN-(β-mercaptoethyl)-2-pyrrolidone;wherein said process produces (i) anoverhead product which contains a smaller volume percentage of said atleast one cycloalkadiene and a larger volume percentage of said at leastone alkadiene than said feed, and (ii) a bottoms product which containssaid solvent and a larger volume percentage of said at least onecycloalkadiene and a smaller volume percentage of said at least onealkadiene than said feed; and wherein said at least one cycloalkadieneis separated from said solvent contained in said bottoms product.
 6. Aprocess in accordance with claim 5, wherein said at least onecycloalkadiene is 1,3-cyclohexadiene and said at least one alkadiene is2-methyl-1,3-pentadiene.
 7. A process in accordance with claim 5,wherein said feed boils at a temperature in the range of about 80° F. toabout 400° F., at atmospheric pressure conditions.
 8. A process inaccordance with claim 5, wherein the weight ratio of said solvent tosaid feed is in the range of about 1:1 to about 40:1.
 9. A process forseparating at least alkadiene containing 4-9 carbon atoms per moleculefrom at least one alkene containing 4-10 carbon atoms per moleculecomprising extractive distillation of a feed which consists essentiallyof said at least one alkadiene and said at least one alkene employing asolvent consisting essentially of a mixture of N-methyl-2-pyrrolidoneand N-(β-mercaptoethyl)-2-pyrrolidone;wherein said process produces (i)an overhead product which contains a smaller volume percentage of saidat least one alkadiene and a larger volume percentage of said at leastone alkene than said feed, and (ii) a bottoms product which containssaid solvent and a larger volume percentage of said at least onealkadiene and a smaller volume percentage of said at least one alkenethan said feed; and wherein said at least one alkadiene is separatedfrom said solvent contained in said bottoms product.
 10. A process inaccordance with claim 9, wherein said at least one alkadiene is1,5-hexadiene and said at least one alkene is 1-hexene.
 11. A process inaccordance with claim 9, wherein said feed boils at a temperature in therange of about 15° F. to about 400° F., at atmospheric pressureconditions.
 12. A process in accordance with claim 9, wherein the weightratio of N-methyl-2-pyrrolidone to N-(β-mercaptoethyl)-2-pyrrolidone insaid solvent is in the range of 1:20 to about 20:1.
 13. A process inaccordance with claim 9, wherein the weight ratio of said solvent tosaid feed is in the range of about 1:1 to about 40:1.
 14. A process forseparating at least one alkadiene containing 4-9 carbon atoms permolecule from at least one alkene containing 4-10 carbon atoms permolecule comprising extractive distillation of a feed which consistsessentially of said at least one alkadiene and said at least one alkeneemploying a solvent consisting essentially of a mixture of cyclohexanoland N-(β-mercaptoethyl)-2-pyrrolidone;wherein said process produces (i)an overhead product which contains a smaller volume percentage of saidat least one alkadiene and a larger volume percentage of said at leastone alkene than said feed, and (ii) a bottoms product which containssaid solvent and a larger volume percentage of said at least onealkadiene and a smaller volume percentage of said at least one alkenethan said feed; and wherein said at least one alkadiene is separatedfrom said solvent contained in said bottoms product.
 15. A process inaccordance with claim 14, wherein said at least one alkadiene is1,5-hexadiene and said at least one alkene is 1-hexene.
 16. A process inaccordance with claim 14, wherein said feed boils at a temperature inthe range of about 15° F. to about 400° F., at atmospheric pressureconditions.
 17. A process in accordance with claim 14, wherein theweight ratio of cyclohexanol to N-(β-mercaptoethyl)-2-pyrrolidone insaid solvent is in the range of about 1:20 to about 20:1.
 18. A processin accordance with claim 14, wherein the weight ratio of said solvent tosaid feed is in the range of about 1:1 to about 40:1.